The diamond filament tether is an unbreakable tether attached to the Planet Express ship. Although it's unbreakable, Farnsworth wants everyone to be careful with it, since it belonged to his grandmother (2ACV12). When Bender tied it to a hook to catch a fish, a Colossal-Mouth Bass took the ship over 6000 feet (1828.8 meters) towards the bottom of the Atlantic Ocean, where they discovered the lost city of Atlanta. Because it is unbreakable, when Fry and Bender flew the ship without permission with the tether attached to the Planet Express headquarters, the ship took part of the building all around the world (2ACV19).